This invention relates to a device for controlling the movement of a movable member such as a table in a machine tool, which is driven by a fluid pressure means, more particularly, to an improvement of a movement controlling device wherein a movable member, which is under control of a fluid pressure means, is made, from a predetermined position of its forward movement stroke, to follow the shifting speed of a member shifted at a cutting feed speed by another driving means such as a motor.
In this kind of device the motor may be of small power, since the movable member is driven by the fluid pressure means and the motor only functions to move a controlling member, such as a rack, for the movable member. And the moving speed of the movable member can also be controlled at a predetermined speed, even when a force be applied in the course of working on the movable member in the forward direction, by means of so controlling the device as to prevent the motor from a rotation due to a force from the rack side, for example, by installing a worm and a worm wheel in an intermediate mechanism between the rack and the motor. Such a device is therefore very much preferable as a controlling device for the movement of a table in a milling machine, especially as one for doing a down cut operation.
As a device having the above purpose, one with the following structure is already known, wherein a rack connected to a one-way-rotatable motor via a one-way clutch is so disposed as to be able to engage with, at a predetermined position in the forward movement stroke of the movable member, the movable member for controlling the moving speed thereof by the movement of the rack regulated by the rotation speed of the motor, and to be able to return to a predetermined position in accordance with the returning movement of the movable member.
In a device of such structure, inherent defects of the clutch itself are liable to appear unrectified, such as an instability of engagement timing, a quick wearing, etc. If and when the engagement (linking) in the one-way clutch is not completed instantaneously, the movable member is liable to be shifted at a high speed for some more time even after the engagement of the movable member and the rack. When the one-way clutch should fail to engage wholely, the movable member would continue to be shifted at the high speed. It can cause the tools for machining a work on the movable member such as drills or cutters to be damaged, furthermore, cause the movable member itself to run desperately, with the result of wrecking the frame or fluid pressure means.
Another proposition, wherein a rack is directly secured to the movable member and the rack is connected to a motor via a gearing mechanism, is also being known. In this case a motor of extremely large power is required, because the motor must directly drive the movable member.